The k-space notation is widely used in the art of MRI to establish a connection between spatial encoding (phase encoding and frequency encoding in the time domain) and the corresponding image obtained by applying the Fourier transform. For data acquisition in the k-space a sampling trajectory of a frequency-encoded signal is typically used. The basic concepts of the k-space notation are explained in more detail in “Principles of Magnetic Resonance Imaging, a Signal Processing Perspective”, Zhi-Pei Liang, Paul C. Lauterbur, IEEE Press Series in Biomedical Engineering, 2000, in particular chapter 5.2.3, pp. 157.
From P. Mansfield, “Multi-planar image formation using NMR spin echoes,” J. Phys. C: Solid State Phys., vol. 10, pp. L55–L58, 1977 an MRI method is known which is commonly referred to as echo-planar imaging (EPI). The term EPI is broadly used to refer to the class of high-speed imaging methods that collect a “complete” set of two dimensional encodings during the free induction decay period following a single excitation pulse. Hence, EPI has become a synonym for single-shot imaging, although multi shot EPI methods with interlaced k-space coverage are also in common use.
A key concept of EPI is the use of time-varying gradients to traverse k-space. For k-space data acquisition a variety of trajectories are known from the prior art such as zig-zag trajectory, rectilinear trajectory and spiral trajectory. For a discussion of the various prior art trajectories reference is made to the above referenced book of Liang and Lauterbur, chapter 9.3, pp. 303.
FIG. 1 and FIG. 2 show rectilinear trajectories for k-space EPI data acquisition.
In a first data acquisition step a first partial data acquisition of the k-space of the target region is obtained by following the trajectory as depicted in FIG. 1. The trajectory starts at the central point 100 of the k-space. From there it goes into the lower left corner 102 of the k-space. Starting from the lower left corner the k-space is partially scanned by means of a rectilinear trajectory. As it is known from U.S. Pat. No. 6,618,607 half of the k-space is covered plus an additional seven lines.
After the first partial k-space acquisition in accordance with FIG. 1 a brief z-shimming gradient pulse z=z0 is applied before the second partial k-space data acquisition is performed in accordance with the trajectory of FIG. 2.
The trajectory of FIG. 2 is also rectilinear and starts at the point in k-space where the trajectory of the partial k-space acquisition of FIG. 1 ends.
A k-space acquisition scheme of the type shown in FIGS. 1 and 2 is known from U.S. Pat. No. 6,618,607 and from “Single-Shot EPI With Signal Recovery From the Susceptibility-Induced Losses”, Allen W. Song, Magnetic Resonance in Medicine 46:407–411 (2001) for application in functional magnetic resonance imaging (fMRI). Based on each of the partial k-space data acquisitions an image is obtained and the two images are overlapped in order to produce a resulting image.
It is an object of the present invention to provide for an improved method for k-space data acquisition in order to increase the spatial sensitivity of MRI.